Some files-generating plugins can generate their files outside of the
default directories being deleted by the clean plugin. You should add
the location of such files in the clean plugin configuration or change
the configuration of those plugins to put their files inside the
project.build.directory which is by default, the target
directory.

For instance, clean could fail if you already have opened a command
line with target as the current dir. Windows locks some ressources and you need
to close the handles on these ressources.
To skip these errors, you could call clean with the command line parameter -Dmaven.clean.failOnError=false.
For more information, refer to Ignoring Errors page.